Laura Gladstone | MIT | USA

The story we expected for neutrino astronomy

Since IceCube was proposed, people have been claiming that you can get a new view of astrophysics by using particles instead of light, and we were pretty sure what the journey would look like. It hasn’t gone quite in the order we expected, but we’re getting that new view of astrophysics, and also, a few years later, filling in the steps we expected to fill first. When we find bits of scientific evidence in a different order than we expected, does that change how excited we get about them?

The sunrise at the South Pole over the IceCube laboratory, the central building on top of the IceCube Neutrino Observatory.

We been expanding astronomy since it started. First, astronomers used telescopes to resolve visible light better. Later, they expanded to different regions of the light spectrum like x-rays and gamma rays. Then, it was a small step to expand from gamma rays, which are easier to think of as particles than as waves, to particles like the atomic nuclei that make up cosmic rays. Neutrinos are another kind of particle we can use for astronomy, and they have unique advantages and challenges.

The hard part about using neutrinos as a messenger between the stars and us is that neutrinos very rarely interact with matter. This means that if thousands pass through our detector, we might only see a few. There are some ways around this, and the biggest trick IceCube uses is to look in a very large volume. If we look for more neutrinos at a time, we have more of a chance of seeing the few that interact. The other trick is that we concentrate on high energies, where the neutrinos have a higher chance of interacting in our detector.

The great thing about using neutrinos as a messenger is that they hardly ever interact, so almost nothing can stop them from arriving at our door. If we see a neutrino in IceCube, it came to us directly from something interesting. We know that its direction wasn’t deflected in any magnetic fields, and it wasn’t dimmed by dust clouds or even asteroid clouds. Every (rare) time we see a high-energy neutrino, it tells us something about the stars, explosions, or black holes that created it.

That’s the story that people like Francis Halzen used to get funding for IceCube originally, and around Madison we still get to hear him tell this story, with his inimitable accent, when he speaks at museums or banquets.

Comparing neutrino astronomy to other new 20th century advances in astronomy, we expected the development of the field to follow a certain story.

We expected that first we would see a “diffuse” signal. This would be part of a large sample including a lot of background events, but some component would only be explained by including astrophysical sources. In IceCube, one of the best ways of reducing background noise is to look for events traveling up through the Earth, since only neutrinos can pass through the Earth. We could also look at high energies, since backgrounds like atmospheric neutrinos fall off exponentially with energy. So we thought the first diffuse astrophysics signal would come from the high-energy tail of an upgoing sample.

After that, we expected to resolve the diffuse sample into some clusters, and after a few of the clusters remained consistent, to declare them astrophysical sources.

What we did instead was to skip to the end of this story. We found astrophysical neutrinos first, and then a diffuse upgoing signal only two years after that (just this past spring). The exciting part about finding this recent diffuse signal isn’t that it’s the first detection of astrophysics, or even the strongest. It’s exciting because it follows the story we thought neutrino astronomy was going to follow.

The first detection was exciting too. That used a different kind of analysis: we identified only a few events (28 in two years) that were extremely likely to be from astrophysical sources. These were so special that each one got a name, using the theme of the Muppets, from Sesame Street and the Muppet Show. One is named Bert, one Ernie, one Mr. Snuffleupagus, one Oscar the Grouch. If we keep analyzing our data this way and eventually get enough events, we can expand to the Muppet Babies cartoons and various muppet movies, even including things like Labyrinth that used Jim Henson’s talents but not the muppets specifically. I’m personally a big fan of the muppet naming scheme, partly because it draws from a cannon recent enough that it includes several women and many kinds of diversity. When naming events is our biggest problem, it will be a great day for neutrino astrophysics. For formal publications, we usually say “HESE” for “High Energy Stating Event,” instead of “muppets.”

The two bedrock assumptions of the muppet analysis were that (1) we’re the most interested in the highest energy events, and (2) the events must have started within the detector; they must be “contained.” That containment requirement means that they must have been neutrinos and not cosmic rays, since comic ray showers contain lots of stuff besides neutrinos that arrives at the same time. We can assume at the highest energies that no cosmic ray could make it through the outer layers of our detector without leaving a trace (unpacked: cosmic rays must leave a trace) but at lower energies some cosmic ray muons can steak through. For the first muppet analysis, we get around this by just looking at the highest energies.

This is backwards from what we expected in two ways: first, the sample we get is mostly from neutrinos coming from above the detector, and second, there are almost no background events in our sample, so we don’t have to include directional clustering to know that we’ve seen astrophysics.

The sample is mostly downgoing because the highest energy neutrinos are blocked by the Earth. Higher energy neutrinos are more likely to interact than low-energy neutrinos; it’s the opposite of our momentum-based intuition from faster cars slamming through walls without stopping. It’s a popular trivium that neutrinos can pass through lightyears of lead without interacting, but that’s only true at low energy scales like the neutrinos from nuclear reactors. At IceCube astrophysics scales, it takes only our tiny planet to stop a neutrino. So the muppet events we do see are mostly ones that don’t pass through the Earth.

Since the muppets sample has almost no background events (at the very most, 10 of the 28, but we don’t know which 10), we don’t need to do a clustering analysis. Traditionally, we thought this was the most promising way to find neutrino point sources, and the background would be neutrinos from interactions in the Earth’s atmosphere. But at PeV energies, there aren’t enough atmospheric neutrinos to explain what we saw, so each event in the new analysis is potentially as interesting as a cluster would be in the old analysis.

We haven’t yet seen clusters using the old techniques, and when we do, it will probably be celebrated by a small party, an email around our collaboration, some nights out for the people involved, and a PhD for someone (or a few someones). But it won’t be the same cover-of-Science-Magazine celebration (that was Mr. Snuffalupagus on the cover) and press coverage that we had for the first discovery. It will be a quiet victory, as it was for the recent diffuse result.

While it doesn’t have to follow the script we expect it to, science can still sometimes choose to follow a familiar plotline. And we are comforted by the familiarity.